Device and method for checking whether a signal with a predetermined frequency is being received

ABSTRACT

A device and method are disclosed, whereby the normally complicated and difficult frequency determination is achieved by simply arranged and executed measures, namely by means of larger, smaller and/or equal comparisons and a counting of certain events. The invention further relates to arrangements whereby the noise signal level, or the influence thereof on the verification to be carried out is reduced.

TECHNICAL FIELD

The present invention relates to a device according to theprecharacterizing clause of claims 1, 5 and 7, and to a method accordingto the precharacterizing clause of claims 15, 16 and 17, i.e. to adevice and to a method for checking whether a signal with apredetermined frequency is being received.

BACKGROUND ART

Such a device and such a method are needed, for example, in theso-called “splitterless integrated voice and data line card”.

Splitterless integrated voice and data line cards are electrical circuitboards which are used in telecommunication switching centers (calledcentral offices hereinafter). They have the responsibility for

-   -   receiving voice data obtained from telecommunication subscribers        (called clients hereinafter), identifying these data as voice        data, converting them into PCM-coded data and forwarding them,        and    -   receiving data obtained via the same line as the voice data and        coming from an XDSL modem existing at the client (called client        modem hereinafter), identifying these data as data coming from a        client modem, converting them into ATM-coded data and forwarding        them.

Splitterless integrated voice and data line cards can be constructed indifferent ways. The splitterless integrated voice and data line cardconsidered at present is a lite version which is standardized in ITUG.lite. ITU G.lite allows parts of the splitterless integrated voice anddata line card to be placed into an energy saving mode in which theyneed much less energy than in normal mode. To wake up the splitterlessintegrated voice and data line card (in order to be able to put thesplitterless integrated voice and data line card back into normal modeif necessary), ITU G.lite defines a so-called wake-up sequence.According to this, the splitterless integrated voice and data line cardmust be brought from the energy saving mode into the normal mode when itreceives a signal with a certain maximum power and one of threepredetermined frequencies from the client modem.

The fact that parts of the splitterless integrated voice and data linecard can be placed into an energy saving mode is found to be of greatadvantage since an extraordinary large number of splitterless integratedvoice and data line cards may be needed in central offices andconsiderable energy saving is thus possible.

Utilization of the possibility of placing parts of the splitterlessintegrated voice and data line card into an energy saving modepresupposes that placing it back into the normal mode functionsreliably.

The main problem here is, in particular, the check as to whether asignal with one of the frequencies at which the splitterless integratedvoice and data line card must be placed back into normal mode is beingreceived.

This is found to be difficult

-   -   because the line between the client modem and the central office        via which the wake-up signal is to be transmitted can be a line        of any length and, in consequence, can have very great        attenuation (e.g. a line with a length of approx. 4 km can        already have approx. −60 dB attenuation),    -   the transmission of the wake-up signal can be disturbed by noise        and by crosstalk,    -   because it can happen that the wake-up signal must pass through        parts of the splitterless integrated voice and data line card        which are in the energy saving mode, as a result of which the        wake-up signal does not arrive with its full power at the device        which has to check whether a wake-up signal is being received.

There are doubtlessly possibilities the use of which makes it possibleto detect reliably whether a wake-up signal is being received even underthese circumstances. However, no possibilities are known which can beimplemented in a simple, small and inexpensive manner and, nevertheless,operate reliably. However, the characteristics lacking in theconventional devices and methods are a very important criterionparticularly because of the very large number of splitterless integratedvoice and data line cards which must be provided.

SUMMARY OF THE INVENTION

The present invention has the object, therefore, of finding a device anda method by means of which it can be checked reliably with littleexpenditure whether a signal with a predetermined frequency is beingreceived.

According to the invention, this object is achieved by the devicesaccording to claims 1, 5 and 7 and by the methods according to claims15, 16 and 17.

The methods and devices according to the invention are distinguished bythe fact that

-   -   the received signal or a signal based on the received signal is        compared with one or more threshold values,    -   selected changes or all changes in the result of the comparison        are counted,    -   the number of changes in the result of the comparison determined        is compared with one or more predetermined values, and    -   depending on the result of this comparison, the device decides        whether a signal with the predetermined frequency is being        received or not (device according to claim 1), and    -   the device determines repeatedly whether a signal with the        predetermined frequency is being received,    -   a device for comparing the sequence of results of the        determination is provided which compares the sequence of results        of the determination obtained during the repeated determination        with one or more predetermined sequences of results of a        determination, and    -   depending on the result of this comparison, the device decides        whether a signal with the predetermined frequency is being        received or not (device according to claim 5), and    -   a control device is provided which, in the case where a signal        with a frequency which is much higher than the predetermined        frequency is apparently being received, makes changes in the        device by means of which the noise signal level and/or its        influence on the check to be performed are reduced (device        according to claim 7), and    -   the received signal or a signal based on the received signal is        compared with one or more threshold values,    -   selected changes or all changes in the result of the comparison        are counted,    -   the number of changes in the result of the comparison determined        is compared with one or more predetermined values, and    -   depending on the result of this comparison, a decision is made        whether a signal with the predetermined frequency is being        received or not (method according to claim 15), and    -   it is repeatedly determined whether a signal with the        predetermined frequency is being received,    -   the sequence of results of the determination obtained during the        repeated determination is compared with one or more        predetermined sequences of results of a determination, and    -   depending on the result of this comparison, a decision is made        whether a signal with the predetermined frequency is being        received or not (method according to claim 16), and    -   in the case where a signal with a frequency which is much higher        than the predetermined frequency is apparently being received,        changes are made in the device carrying out the check by means        of which the noise signal level and/or its influence on the        check to be performed are reduced (method according to claim        17).

The claimed devices and the devices needed for carrying out the claimedmethods can be largely constructed in digital technology, as a result ofwhich they can be implemented in a relatively small and simple manner.

In the claimed devices and methods, in addition, measures are taken bymeans of which the risk of wrong decisions is reduced to a minimum.

In consequence, the claimed devices and methods make it possible, bothby themselves and in combination with one another, reliably to checkwith little expenditure whether a signal with a predetermined frequencyis being received.

Advantageous further developments of the invention can be found in thesubclaims, the following description and the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In the text which follows, the invention will be explained in greaterdetail by means of illustrative embodiments and referring to thefigures, in which:

FIG. 1 shows a first device for checking whether a signal with apredetermined frequency is being received,

FIG. 2 shows a second device for checking whether a signal with apredetermined frequency is being received, and

FIG. 3 shows a third device for checking whether a signal with apredetermined frequency is being received.

DETAILED DESCRIPTION OF THE INVENTION

The devices described in the text which follows, and the methodsdescribed in the text which follows, are optimized for use in the“splitterless integrated voice and data line card” already mentionedinitially; in the example considered, the devices are a part of asplitterless integrated voice and data line card. However, there is norestriction to this. The devices and the methods can also be used in anyother systems in which it is necessary to detect the reception of asignal with a predetermined frequency.

In FIG. 1, a first illustrative embodiment of a device is shown by meansof which it is possible to check whether a signal with a predeterminedfrequency is currently being received.

The signal with the predetermined frequency is the wake-up signal whichis output by the client modem, if required, in order to place at leastparts of the splitterless integrated voice and data line card back intonormal mode from the energy saving mode.

The wake-up signal to be detected is designated by the reference symbolWUS. In the example considered, the wake-up signal can have threedifferent frequencies. The device shown in FIG. 1 first checks whether awake-up signal having the first wake-up frequency is being received,then checks whether a wake-up signal having the second wake-up frequencyis being received and finally checks whether a wake-up signal having thethird wake-up frequency is being received; these checks are repeatedcontinuously.

The wake-up signal WUS first passes into a band-pass filter BPF. Theband-pass filter BPF, or more precisely the position of the frequencyband passed by it, can be adjusted via a control connection fOselect. Inthe example considered, three frequency bands can be set which aredifferent from one another. The different frequency bands are selectedin such a manner that either a wake-up signal having the first wake-upfrequency or a wake-up signal having the second wake-up frequency or awake-up signal having the third wake-up frequency is passed.

The output signal of the band-pass filter BPF is supplied to anamplifier AMP. This amplifier amplifies the signal supplied to it.

The output signal of the amplifier AMP is supplied to a high-pass filterHPF. This is provided for filtering out the noise generated in theamplifier AMP, particularly the so-called flicker noise component. Saidflicker noise component is attenuated by the high-pass filter HPF tosuch an extent that it does not have much influence on the furtherprocessing of the signal. In addition, the high-pass filter HPF alsoeliminates any offset which may be present.

The output signal of the high-pass filter HPF is supplied to acomparator COMP. The comparator COMP operates with hysteresis, i.e.

-   -   the comparator COMP outputs an output signal with a first level        if the potential of the signal supplied to it exceeds a first        (upper) threshold value, and retains this state until the        potential of the signal supplied to it drops below a second        (lower) threshold value, and    -   the comparator COMP outputs an output signal with a second level        if the potential of the signal supplied to it drops below the        second (lower) threshold value, and retains this state until the        potential of the signal supplied to it exceeds the first (upper)        threshold value.

The threshold values are set in such a manner that the noise which isstill contained in the signal supplied to the comparator COMP cannotresult in any switch-over of the output signal of the comparator COMP,that is to say that the threshold values are above and below the noisesignal level. If the device shown in FIG. 1 is supplied with a signalWUS exhibiting one of the wake-up frequencies and the band-pass filterBPF allows this signal to pass, a squarewave signal exhibiting therelevant wake-up frequency is obtained at the output of the comparatorCOMP; if the device is supplied with a signal having a frequency whichis not passed by the band-pass filter BPF or if the device is notsupplied with a signal, the output signal of the comparator COMP remainspermanently at the 0 level or the 1 level.

The output signal of the comparator COMP is supplied via a switch S to acounter COUNT which it triggers. The counter COUNT thus counts thenumber of changes in the output signal of the comparator COMP from lowlevel to high level (or the number of changes in the output signal ofthe comparator COMP from high level to low level). The countingfrequency thus corresponds to the frequency of the wake-up signal WUS.

The counter COUNT can “only” count when and as long as the switch S isclosed; when the switch S is opened, the counter does not receive atrigger signal which causes to it increment. The switch S is normallyopened and is in each case only closed for a quite particular time.Before the switch S is closed, the counter is reset by application of acorresponding signal to a reset terminal reset of the counter. If theswitch S is then closed for the determined time, the counter counts at acounting frequency corresponding to the frequency of the wake-up signalWUS during this time. The counting ends when the switch S is opened.

The count reached after the switch S has been opened is supplied to acomparison unit CU which compares it with one or more values which arestored in a look-up table provided in the comparison unit or somewhereelse.

The values stored in the look-up table are selected in such a mannerthat when the count of the counter COUNT corresponds to one of thesevalues, it can be assumed that a wake-up signal having a wake-upfrequency is being received.

The circumstance that the count of the counter COUNT is compared notonly with one value but with a number of values makes it possible todecide that a signal with one of the wake-up frequencies is beingreceived even

-   -   if the frequency of the received signal does not precisely        correspond to one of the wake-up frequencies, and/or    -   if disturbances during the transmission or during the evaluation        have led to the count of the counter differing to a greater or        lesser extent from the count which would have been reached in        the undisturbed case.

As an alternative, it can be provided to define in the look-up table oneor more value ranges within which the count of the counter COUNT must belocated in order to assume that a wake-up signal having a wake-upfrequency is being received.

The comparison unit CU and/or the look-up table have a controlconnection, not shown in FIG. 1, via which it is possible to set thevalue or values with which the count of the counter COUNT is to becompared. These values differ for the different wake-up frequencies. Asan alternative, it can be provided to close the switch S, depending onthe wake-up frequency for which the received signal is being examined,in each case for a length of time such that the same count is obtainedfor all wake-up frequencies.

The comparison unit CU outputs a signal representing the result of thecomparison. At the same time, this signal signals whether a wake-upsignal having one of the wake-up frequencies is currently being receivedand can be used as a wake-up bit WUB which causes the parts of thesplitterless integrated voice and data line card, placed in the energysaving mode, to be placed back into the normal mode.

Using the device described above it is possible in a surprisingly simplemanner to check whether a wake-up signal having one of the wake-upfrequencies is being received. In particular, it has been possible toimplement the frequency determination, which is normally very expensiveand complicated, by measures which can be achieved and performed in acomparatively simple manner, namely by greater-than, less-than and/orequal-to comparisons and by counting certain events; lastly, “only” thezero transitions of the received signals are detected, counted andcompared with nominal values or nominal value ranges. In spite of thesimple configuration of the device shown in FIG. 1, it operates veryreliably. In particular, it is possible, by comparing the count of thecounter COUNT with a number of nominal values or a range of nominalvalues, to ensure that disturbances in the transmission and theevaluation of the wake-up signal do not have a negative influence on theresult of the check as to whether a wake-up signal having one of thewake-up frequencies is currently being received. To this is added thefact that the counted events, namely the number of zero transitions ofthe received signal per unit time, are not disturbed like othercharacteristics (for example the amplitude variation) of the receivedsignal during the transmission and/or that existing disturbances can beeliminated, for example, by means of the zero transition detector (thecomparator COMP) operating with hysteresis and/or in another manner withrelatively little expenditure.

A device which operates even more reliably is shown in FIG. 2.

Compared with the device in FIG. 1, the device shown in FIG. 2 isdistinguished by the fact that the decision whether a signal having oneof the wake-up frequencies is being received or not is made on the basisof a sequence of greater or lesser length of events which are obtainedduring a repeated determination, carried out as in the device accordingto FIG. 1 or differently, as to whether a signal having one of thewake-up frequencies is being received or not.

In the example considered, the device shown in FIG. 2 contains in itsentirety the device shown in FIG. 1 and described with referencethereto, but additionally has a digital evaluation logic (provided afterthe comparison unit CU); in the example considered, this digitalevaluation logic consists of a shift register SR and a second comparisonunit (a comparison unit for the sequence of determination results) CU2.

Thus, unlike the device according to FIG. 1, it is not the output signalof the comparison unit CU which is used as output signal of the device.Instead, a decision is made (by the shift register SR and the secondcomparison unit CU2), in dependence on the variation of the outputsignal of the comparison unit CU, whether a signal having one of thewake-up frequencies is currently being received.

The output signal of the comparison unit CU is supplied to the shiftregister SR and transferred into the latter with a clock pulse ‘clock’and shifted through. The clock pulse ‘clock’ preferably has the samefrequency as the signal used for controlling the switch S. This storesthe variation with time of the output signal of the comparison unit CUin the shift register SR.

The content of the shift register SR is supplied to the secondcomparison unit CU2 and compared in the latter with one or morereference bit patterns refpat also supplied to the second comparisonunit CU2. The second comparison unit CU2 has a control terminal viawhich it is possible to set the reference bit pattern or bit patternswith which the content of the shift register is to be compared. Thesignal fOselect controlling the band-pass filter BPF can be applied tothis control terminal. This is necessary because the data contained inthe shift register SR are the result of successively conducted differentchecks; it is necessary to use different reference bit patterns refpatin dependence on the frequency the reception of which is currently to bechecked by the second comparison unit CU2.

The comparison unit CU2 outputs a signal representing the result of thecomparison. At the same time, this signal signals whether a wake-upsignal having one of the wake-up frequencies is currently being receivedand, therefore, can be used as the wake-up bit WUB which initiates theresetting into the normal mode of the parts of the splitterlessintegrated voice and data line card placed in the energy saving mode.

The reference bit patterns are selected in such a manner that a decisionthat a signal having one of the wake-up frequencies is being received isonly made if this has been determined by the comparison unit CU apredetermined number of times in succession and/or a predeterminednumber of times within a predetermined period of time.

This makes it possible to prevent a decision, that a signal with one ofthe wake-up frequencies is currently being received from being made onthe basis of a disturbance which has the accidental consequence that theoutput signal of the comparison unit CU wrongly signals the reception ofa signal with one of the wake-up frequencies.

Thus, the device for checking whether a signal with a predeterminedfrequency is being received, shown in FIG. 2 and described withreference thereto, operates even more reliably than the device shown inFIG. 1 and described with reference thereto.

An even more reliable check as to whether a signal with a predeterminedfrequency has been received is possible by means of the device shown inFIG. 3.

The device shown in FIG. 3 contains the device shown in FIG. 2 anddescribed with reference to FIGS. 1 and 2, but additionally has adigital adaptation logic which, taking into consideration the givenconditions, adjusts the amplifier AMP in such a manner that the check asto whether a wake-up signal is being received cannot be disturbed bynoise or other disturbances under any circumstances; in the exampleconsidered, this digital adaptation logic consists of a storage device,formed by a flip-flop FF in the present case, and an amplificationadjusting device AMPCTL.

The flip-flop FF is connected to a 0 V connection of the counter COUNTindicating an overflow of said counter. It buffers the signal outputfrom the 0 V connection of the counter COUNT and forwards it to theamplification adjusting device AMPCTL.

Depending on the value of the signal supplied to it from the flip-flopFF, the amplification adjusting device AMPCTL changes the gain of theamplifier AMP; it reduces the gain when the signals supplied to it fromthe flip-flop FF indicates an overflow of the counter and/or itincreases the gain when the signal supplied to it from the flip-flop FFdoes not indicate an overflow of the counter.

This procedure is based on the finding that the counter COUNT can onlyoverflow if the signal output from the amplifier AMP exhibits too muchnoise or, more precisely, too high a noise signal level. A high noisesignal level has the effect that the output signal of the comparatorCOMP changes much more frequently than would be the case if thecomparator were supplied with a signal exhibiting no noise or onlylittle noise (a low noise signal level) . When the noise signal levelexceeds the threshold values of the comparator, the output signal of thecomparator changes so frequently that the counter COUNT triggered bythis signal overflows within the time during which the switch S isclosed. Naturally, the prerequisite for this is that a counter is used,the maximum count of which is not very much higher than the count whichis reached when the comparator is supplied with a noiseless signalexhibiting one of the wake-up frequencies or, respectively, that acounter is used which overflows when the comparator is supplied with asignal with a noise signal level which exceeds the threshold values ofthe comparator; providing such a counter does not have any negativeeffects on the operation and the possibility of implementing the deviceand it can therefore be provided without problems. The overflow of thecounter COUNT is an unambiguous indication that the signal supplied tothe comparator COMP exhibits so much noise that reliable detection of asignal exhibiting one of the wake-up frequencies is no longer possible.However, the noise of the signal supplied to the comparator COMP can bereduced by reducing the gain of the amplifier AMP. With a lower gain,the noise component contained in the signal supplied to the splitterlessintegrated voice and data line card is amplified less and, in addition,an amplifier AMP amplifying with a lower gain also generates less noiseitself. Thus, adjusting the gain of the amplifier AMP in dependence onthe count of the counter COUNT is a very simple and yet extremelyeffective possibility of adjusting the gain of the amplifier to theideal value.

There are various possibilities of how the optimum gain can bedetermined and adjusted.

A first possibility consists in first setting the gain of the amplifierAMP to the maximum value. If the noise signal level is too high, anoverflow occurs at the counter COUNT. This causes the amplificationadjusting device AMPCTL to reduce the gain. If then (with the reducedgain) another overflow of the counter occurs, the amplificationadjusting device AMPCTL again reduces the gain. This process is repeateduntil a gain adjustment has been reached at which there is no longer anoverflow of the counter. This gain setting or a gain setting which hasbeen reduced by another one or more steps as a safety margin is then theideal gain setting which is preferably used during the check as towhether a signal with a predetermined frequency is being received, andsupplies the most reliable results.

As an alternative, it would also be conceivable initially to set thegain of the amplifier AMP to the minimum value and then to increase ituntil the counter overflows and then to reduce it again by one or moresteps.

It would also be conceivable initially to set the gain of the amplifierAMP to a value which is between the minimum value and the maximum valueand, starting from there, to reduce the gain step by step (if thecounter indicates an overflow) or to increase it step by step (if thecounter does not indicate an overflow).

It should be clear that it is also possible to proceed in any other waythan has been described above in determining and setting the optimumgain. In particular, it is also possible arbitrarily to define and/orvary, among other things, the size and/or the number of steps by whichthe gain factor is changed.

In general, it is found to be advantageous if the amplifier AMP has anumber of amplifier stages, the gains of which can be changed in stepsof different sizes. In the example considered, the amplifier AMPcomprises two amplifier stages, designated by the reference symbols AMP1and AMP2 in FIG. 3, where the gain of the first amplifier stage AMP1 canbe changed in large steps and the gain of the second amplifier stageAMP2 can be changed in comparatively small steps.

A gain adjustment made as described or differently makes it possible toadapt the splitterless integrated voice and data line card optimally tothe respective situations, particularly to the length of the linebetween the client modem and the central office (the attenuation) andthe existing interference.

It should be clear that it is not mandatory to use the overflow of thecounter COUNT as the triggering event for the measures described. As analternative, it is also possible to use the reaching or exceeding of aparticular different count as the triggering event. The reaching orexceeding of the particular count can be detected by monitoring thecount output by the counter or selected bits of the count (preferablythe most significant bits).

The same result or a similar result can be achieved if, additionally oras an alternative, the band-pass filter BPF, the high-pass filter HPFand/or the comparator (for example the threshold values used by thelatter) are influenced in dependence on the count of the counter reachedor on other parameters providing information about the noise signallevel or other interfering influences.

The devices and methods described make it possible, independently of thedetails of the practical implementation, with little expenditure but,nevertheless, extremely reliably, to check whether a signal with apredetermined frequency is being received.

If, as in the example considered, it must be checked whether thereceived signal exhibits one of a number of different frequencies, itmay prove to be advantageous if the device described also suppliesinformation about which of the frequencies to be detected are exhibitedby the received signal. This information, too, can be obtained from thedevices described: the comparison units CU and CU2 can detect which ofthe frequencies to be detected are exhibited by the received signal fromthe result of the comparison (from the knowledge of which of thecomparisons to be performed produced an agreement)—if necessary, takinginto consideration the current or preceding value of the control signalf0select.

1. Device for checking whether a signal with a predetermined frequencyis being received, wherein: (a) the received signal or a signal based onthe received signal is compared with one or more threshold values bymeans of a comparison unit, (b) selected changes or all changes in theresult of the comparison are counted by means of a counting unit, (c)the number of changes in the result of the comparison determined iscompared with one or more predetermined values, (d) depending on theresult of this comparison, a decision unit decides whether a signal withthe predetermined frequency is being received or not, (e) a controldevice is provided which, in the case where a signal with a frequencywhich is much higher than the predetermined frequency is apparentlybeing received, makes changes in the device by means of which the noisesignal level and/or its influence on the check to be performed arereduced, and (f) the counting device overflows when the comparisondevice is supplied with a signal with a noise signal level exceeding thethreshold values of the comparison device.
 2. Device according to claim1, wherein the comparison of the received signal or of the signal basedon the received signal with one or more threshold values is made by acomparator, (a) which outputs an output signal exhibiting a first levelif the potential of the signal supplied to it exceeds an upper thresholdvalue, (b) which outputs the output signal exhibiting the first leveluntil the potential of the signal supplied to it drops below a lowerthreshold value, (c) which outputs an output signal exhibiting a secondlevel if the potential of the signal supplied to it drops below thelower threshold value, and (d) which outputs the output signalexhibiting the second level until the potential of the signal suppliedto it exceeds the upper threshold value.
 3. Device according to claim 2,wherein the changes in the result of the comparison are counted by acounter triggered by the output signal of the comparator.
 4. Deviceaccording to claim 3, wherein the output signal of the comparator is ineach case only forwarded to the counter for a predetermined time. 5.Device for checking according to claim 1, wherein: (a) the devicedetermines repeatedly whether a signal with the predetermined frequencyis being received, (b) a device for comparing the sequence of results ofthe determination is provided which compares the sequence of results ofthe determination obtained during the repeated determination with one ormore predetermined sequences of results of a determination, and (c)depending on the result of this comparison, the device decides whether asignal with the predetermined frequency is being received or not. 6.Device according to claim 5, wherein the results obtained in therepeated determination as to whether a signal with the predeterminedfrequency is being received are written into a shift register.
 7. Deviceaccording to claim 1, wherein the change consists in influencing anamplifier for amplifying the signal to be detected, which amplifier iscontained in the device.
 8. Device according to claim 7, wherein thegain of the amplifier is reduced.
 9. Device according to claim 1,wherein the change consists in influencing a filter for filtering thesignal to be detected, which filter is contained in the device. 10.Device according to claim 1, wherein the change consists in influencinga comparator for comparing the signal to be detected with one or morethreshold values, which comparator is contained in the device. 11.Device according to claim 10, wherein the threshold value or thresholdvalues with which the signal to be detected is compared are changed. 12.Device according to claim 1, wherein the changes are made when acounter, provided for determining the frequency of the received signal,exceeds a predetermined count.
 13. Device according to claim 12, whereinthe changes are made when the counter overflows.
 14. Method for checkingwhether a signal with a predetermined frequency is being received,wherein: (a) the received signal or a signal based on the receivedsignal is compared with one or more threshold values by means of acomparison unit, (b) selected changes or all changes in the result ofthe comparison are counted by means of a counting device, (c) the numberof changes in the result of the comparison determined is compared withone or more predetermined values, and (d) depending on the result ofthis comparison, a decision is made by means of a decision unit whethera signal with the predetermined frequency is being received or not, (e)in the case where a signal with a frequency which is much higher thanthe predetermined frequency is apparently being received, changes aremade in the device carrying out the check by means of which the noisesignal level and/or its influence on the check to be performed arereduced, and (f) the counting device overflows when the comparisondevice is supplied with a signal with a noise signal level exceeding thethreshold values of the comparison device.
 15. Method for checkingaccording to claim 14 whether a signal with a predetermined frequency isbeing received, wherein: (a) it is repeatedly determined whether asignal with the predetermined frequency is being received, (b) thesequence of results of the determination obtained during the repeateddetermination is compared with one or more predetermined sequences ofresults of a determination, and (c) depending on the result of thiscomparison, a decision is made whether a signal with the predeterminedfrequency is being received or not.